This is for sure beginner friendly. I did a drift dive down the Rainbow River as my first dive out of Open Water class a few years ago, and it was plenty simple. I did not use a charter and did not start up in the state park (we used a family member's boat and started as close to the park as we could with a private boat), so we did have to tow a dive flag and figure things out ourselves. It is very shallow and the current is manageable, but depending on what part of the river you are in, be very careful about other boats (especially with the water being so shallow), and be careful not to get yourself tangled up in the small, private docks (with current, it can be difficult to get untangled). Using a dive operator, I am sure the dive is even easier and more beginner friendly. We saw a mix of fish, freshwater plants, and some turtles. I think my average depth was 8ft?
The viz was as good as the articles show, however I imagine after a solid rain, the viz does drop significantly. There are a few places where the river connects with other water sources heavy with tannins, so around those merges, the viz drops due to the mixing of the tea colored water. You also want to go on a sunny, and not cloudy day as it helps the viz out a lot.